228 STUDY OF BRAINS OF SIX EMINENT SCIENTISTS AND SCHOLARS. 



which impulses are carried. The curious fact to be remembered in this connection is 

 that the function of nerve-fibers within the brain is only established when the medul- 

 lary sheath has developed. But this development of mature nerve-fibers does not oc- 

 cur simultaneously throughout the brain, but step by step in a definite order of suc- 

 cession ; equally important bundles of fibers are developed (medullated) simultane- 

 ously, but those of dissimilar importance develop one after another in accordance with 

 a biological law recently formulated by Professor Flechsig. This successive medulla- 

 tion of biuidles of fibers going to the various areas of the cortex closely corresponds to 

 the successive awakenings of mental activities and faculties in the growing child. Now 

 whether a given child shall be normal, backward or precocious depends largely, if not 

 wholly, upon this progressive ripening of the numerous nerve-elements of the brain. 

 When the maturing process is a slow one and the stimulating training of ordinary 

 educational methods finds only slight response, the child remains backward and may 

 ever be feeble-minded. Contrariwise, the rapid and early development of a brain that 

 is generously planned to begin with often results in a mental superiority that is only 

 found in the precocious genius. Why some brains develop slowly and others rapidly 

 is another question to be relegated to the consideration of the " inequality of man." 

 In the precocious genius it must for the present be assumed that the ripening of the 

 nerve-fibers is perhaps stimulated by some obscure bio-chemical conditions which are 

 less marked or less eflfectual in the ordinary child. It is fair to assume some such 

 chemical factor, for the absence of impairment of function of the thyroid gland, which 

 is invariably associated with mental failure and retention of the infantile state so 

 characteristic of sporadic cretinism, cannot be disregarded in this connection. We 

 need not assume that the secretions of the thyroid gland alone are essential ; many 

 other substances, as yet undiscovered, may be as necessary. Who knows whether 

 there may not be some substance wliich stimulates brain-development just as the 

 adrenal secretion stimulates the unstriped muscle cells of the arterial system to con- 

 tract. Indeed, the early ripening of the brain sometimes seems to be an expression of 

 over-stimulation by some substance either in itself toxic or produced in abnormal 

 quantity or strength. It is suggestive that some infant prodigies fail to uphold them- 

 selves beyond the age of pubert}' and usually fall prey to the ravages of tuberculosis 

 or other constitutional diseases. 



The history of the latest epoch of animal life upon this planet is the history of 

 the development of man's progressive brain. The attainment of the erect attitude by 

 Pithecanthropus, our direct ancestor, the gradual acquisition of reasoning and ideation 

 as well as manual skill were the chief factors in bringing about the superior structure 

 of the human brain. Perhaps the most important stimulus to brain-development was 



